The present invention relates to a device for applying a coating liquid to the surface of a moving carrier, and more particularly to an improved device for applying a coating liquid to the surface of a moving carrier through the use of the surface of a doctor edge portion of the application device.
Various conventional devices are known for applying a magnetic liquid or a photographic photoconductive liquid to a moving web. In one of these devices, the liquid is applied to a flexible carrier while the carrier is pinched between the application surface of the device and the carrier. Examples of such devices include an extrusion-type application device, a gravure-smoother-type application device and a doctor-type application device.
The flexible carrier referred to herein can be a flexible sheet or web such as a plastic film, paper, polyolefin-coated paper, or a sheet of a metal such as aluminum and copper. A liquid such as a magnetic liquid, photographic photosensitive liquid, or various other liquids is applied to the carrier to manufacture a magnetic recording material, a photographic film, printing paper or the like.
For example, the application device can be an extrusion-type application device having a doctor edge portion, such devices typically being used for applying a magnetic dispersion liquid. Examples of such a device, which is widely used for applying magnetic dispersion liquids, are described in the Japanese Unexamined Published Patent Applications Nos. 138036/75 and 84771/80 and the Japanese Patent Application No. 7306/79.
As depicted in FIG. 6, the clearance t between the surface 5a of the doctor edge portion 5 of such an extrusion-type application device and a flexible carrier W is generally set to decrease toward the downstream end A of the surface 5a of the doctor edge portion so that the clearance is a minimum value at the downstream end. As a result, the carrier W is moved at an appropriate angle over the surface 5a of the doctor edge portion 5 so that the pressure of the liquid F is made appropriate as the liquid is applied to the carrier. Since the clearance t is a minimum at the downstream end A of the surface 5a of the doctor edge portion 5, the shearing stress of the liquid F is made to concentrate at the downstream end, exerting strong force against the surface 5a of the doctor edge portion 5.
Since the magnetic dispersion liquid contains materials o such as a magnetic substance, an abrasive and a lubricant, which all have a tendency to corrode or wear metals, the surface 5a of the doctor edge portion 5 of the extrusion-type application device deteriorates more rapidly at the downstream end A of the surface than at other parts of the surface. The resulting nonuniformity in the surface 5a leads to problems such as streaking of the liquid applied to the carrier W. Streaks in the manufactured recording media cause various problems, such as a deteriorated S/N or C/N ratio. For the above reasons, the doctor edge portion must often be replaced or reprocessed. The problem is more serious when the liquid is applied as a thin film or applied at a high rate.